The overall goal of this research program is to understand the acquisition of motor skills in infancy. The proposed studies are informed by a dynamic systems perspective. Thus, the research strategy is twofold: (1) to provide a detailed description of the developmental changes in reaching and walking and their multiple interacting subsystems, and (2) to identify and experimentally manipulate agents and processes critical to the emergence of new behavioral forms. Specific aim I is to complete the analysis of longitudinal data on the kinematics, kinetics, and muscle patterns of infant reaching, collected weekly from 4 infants from weeks 3 - 52. This will describe the transition from spontaneous to goal-directed reaching, the improvements in the accuracy, speed and smoothness in reaching, and the dynamic segmental forces and muscle patterns that underlie these changes. Specific aim 2 is to conduct a longitudinal study of infants between 6 months and 18 months to trace the kinematic, kinetic and EMG characteristics of treadmill steps, supported steps, and independent walking. This study will describe the developmental transition in the neuromuscular precursors of walking when infants stand, bear weight, and move forward independently. Specific aim 3 is to conduct experiments where kicking and reaching movements are perturbed by a slight tug, which simulates the naturally occurring internal and external force environment. This study investigates how infants detect and adjust their muscles to proprioceptive input. Specific aim 4 is a series of experiments investigating how infants acquire new motor forms. Infants will be trained to use bilateral kicks to activate a mobile, and the conditions and contexts that facilitate learning and remembering the new pattern of coordination will be manipulated. These studies are important to (1) illuminate basic developmental processes; (2) understand fundamental mechanisms of motor coordination and control; and (3) impact on clinical practice, especially physical and occupational therapy with children.